The phrase "was onto someone" is often used to indicate that someone suspects or has discovered something about another person's actions, intentions, or motives. There are several synonyms for this phrase, including "suspecting," "doubting," "challenging," "questioning," "investigating," and "revealing." Each of these words can be used to express a slightly different shade of meaning, depending on the context and the degree of certainty or suspicion involved. For example, "investigating" suggests a more deliberate and systematic effort to uncover the truth, while "questioning" implies a more tentative or exploratory approach. Ultimately, the choice of synonym depends on the writer's intended tone and message.